A construction of a prior example concerning such kind of an electromagnetic induction heating apparatus will be described based on FIG. 6.
The electromagnetic induction heating apparatus comprises a pipe 11 formed of an insulator, a heating element 12 in a fluid introduced in the pipe 11 and a coil 13 rolled around the pipe 11, and generates heat by an electromagnetic induction caused between the heating element 12 and the coil 13. A high frequency inverter 5 with a sensorless high power factor is used as the high frequency electric current generator for making an alternating field on the coil 13. A controller 3 controls an output of the high frequency electric current generator, which includes a phase shift controling device 3a and a gate driver 3b. A temperature sensor 17 is fixed on the side of outlet of the pipe 11. A temperature regulator 2 is connected with the phase shift controling device 3a. In addition, the high frequency electric current generator includes a rectification section 22 for an alternating current power supply 21, a non-smoothing filter 23 and an inverting section 24.
Normal functions of the above mentioned electromagnetic induction heating apparatus will be described. At the time of ordinary running, a low-temperature fluid 14 is supplied from a lower side of the pipe 11. The low-temperature fluid 14 turns into a fluid 15 of the turbulent flow within the heating element 12. The fluid 15 of the turbulent flow directly exchanges heat with the heating element 12 and turns into a high-temperature fluid.
The high-temperature fluid is discharged from an upper side of the pipe 11. A temperature of the high-temperature fluid is measured by the temperature sensor 17. An instruction corresponding to a difference between the high-temperature fluid and a required temperature is output from the temperature regulator 2 to the phase shift controling device 3a. The instruction is input in the high frequency electric current generator 5 through the gate driver 3b. Then, an electric current from the high frequency electric current generator 5 is appropriately controlled. According to the direct heating type of the electromagnetic induction heating apparatus 1, a high heat conductive efficiency is achieved by making heat conductive areas of the heating elements 12 large. Hence, the time when temperature raises up till a required temperature can be shortened. In other words, a starting-up time for starting heating at the required temperature can be shortened.
The heating element 12 generates heat itself by electromagnetic induction then heats up a fluid directly. Therefore, special material is used for the heating element 12. The material of the heating element required to be a ferromagnet for generating heat by electromagnetic induction, and, to be efficient in corrosion resistance since it directly contacts with the fluid. As the material which satisfies the above conditions, there is a stainless steel of martensite group which has Cr,Fe as the principal ingredient and is proposed in a patent application No. H6-297287.
In order to further shorten the starting-up time for starting heating at the required temperature, it is proposed to heat the entire electromagnetic induction heating apparatus in advance. In this regard a fluid is filled in a pipe 11, and a heating element 12, a temperature sensor 17 and so on are in the fluid. The pipe 11, the heating element 12 and the fluid are preheated when the high frequency electric current generator 5 operates.
A significant difference results between a fluid temperature detected by the temperature sensor 17 and a real temperature of the heating element, since the fluid does not flow in the pipe 11 during the preheating. Also, an excessive electric current has been flowing into the high frequency electric current generator 5 in spite of the real temperature of the heating element 12 rising up more than the required temperature. Consequently, the electromagnetic induction heating apparatus is damaged.
A heating system which includes the coil 13 and the heating element 12, can be shown by a model of a transformer circuit whose leakage inductance is large, and can be displayed in a simple R-L circuit which includes L1 and R1. In the R-L circuit, an equivalent resistance of the R-L circuit is defined by R, a specific resistance of the R-L circuit is defined by r, an electric current of the R-L circuit is defined by I.sub.C and an electric power of the R-L circuit is defined by P.sub.0. FIG. 7 illustrates how the R,r,I.sub.C and P.sub.0 change with the change concerning the temperature of the heating element 12.
In FIG. 7, the specific resistance r rises when the temperature increases while the equivalent resistance suddenly decreases from a certain temperature T.sub.0. Accordingly, the electric current I.sub.C and the electric power P.sub.0 turn to increase from the certain temperature T.sub.0. When the electric current I.sub.C and the electric power P.sub.0 increase to excess of and surpass a rating, some electric power elements of the high frequency electric current generator 5 are damaged. This phenomenon occurs as follows. Temperature of the heating element 12 becomes high and reaches a magnetic transformation temperature of the magnetic material forming the heating element 12. In other words, when the temperature of the heating element 12 reaches the magnetic transformation temperature T.sub.0 of the magnetic material forming the heating element 12, a magnetism of the heating element 12 suddenly changes from ferromagnetic to paramagnetic. Consequently, the coil 13 becomes a short circuit condition.
Therefore, it is proposed that a chemical composition of the heating element 12 be changed so as to shift the magnetic transformation temperature T.sub.0 to a high-temperature side. However, when the chemical composition of the heating element 12 is changed, a new problem occurs in that the corrosion resistance of the heating element 12 gets worse. Moreover, the temperature sensor 17 does not directly detect the temperature of the heating element 12 and detects the temperature of the fluid which has exchanged heat with the heating element 12. Accordingly, a reckless driving of the high frequency electric current generator 5 can not be avoided when the fluid 14 is heated without any stream of the fluid 14.